The present invention relates to superheterodyne radio receivers and more particularly to systems for suppressing spurious signals in superheterodyne receivers.
In accordance with the operational fundamentals of superheterodyne receivers, communications input signals across frequency bands of interest are mixed with a tuneable local oscillator signal to produce a constant intermediate frequency signal. However, because the mixers employed in this process are inherently non-linear devices harmonics of the input signals and of the local oscillator signal are generated during the mixing process and are likewise mixed resulting in signal products having frequencies as defined by expression 1 below: EQU .+-.(Nxf.sub.RF).+-.(Mxf.sub.LO) (1)
where:
N and M are zero or positive integers, PA1 f.sub.RF is the frequency of the radio frequency communications input signal, and PA1 f.sub.LO is the frequency of the local oscillator signal to be mixed with the input signal.
Depending upon the input frequencies of interest, the intermediate frequency chosen and the local oscillator frequency selected (i.e. the conversion scheme for the receiver), certain spurious products generated pursuant to the mixing process may fall within or close to the intermediate frequency passband for the receiver thereby creating undesirable interference with the communication signal.
Superheterodyne receivers are frequently subject to a separate problem in that the local oscillator signal, which is usually a high level signal, is passed through the mixer and is radiated out through the receiver's antenna. Although the local oscillator signal reaching the antenna is attenuated to the extent of the mixer isolation, nevertheless, enough radiation can leak through to cause troublesome interference problems for nearby receivers. Furthermore, sufficient signal energy can occasionally be leaked out to allow the receiver's location to be identified which can be highly undesirable in some military applications.
These problems have been dealt with in the past by making good choices for the frequency conversion scheme used in the receiver, providing mixers having good balance and isolation and filtering the input to the mixer. However, in wideband applications it is difficult to resolve these problems using these techniques and the solutions which can be obtained are only partially effective.
It is therefore an object of the present invention to provide a system for suppressing spurious product signals arising during the mixing process in superheterodyne radio receivers.
It is another object of the present invention to provide a system for reducing the amount of radiation emitted from superheterodyne receivers due to leakage of the local oscillator signal through the mixer to the receivers input antenna.
It is a further object of the present invention to provide a system for suppressing mixer generated spurious product signals and local oscillator leakage radiation which is effective yet is simple and economical to implement.